A semiconductor device capable of holding data that includes a transistor whose semiconductor layer is formed using silicon (Si) and a transistor whose semiconductor layer is formed using an oxide semiconductor (OS) has attracted attention (see Patent Document 1).
In recent years, with the increase in the amount of data manipulated, a semiconductor device having a large storage capacity has been required. In such situations, the semiconductor device disclosed in Patent Document 1 has a structure in which multilevel data is stored and read.